For the past several years, extensive research has been devoted to the development of metal matrix composites, such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal matrix composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. However, such composites typically suffer from poor ductility and fracture toughness.
Prior art techniques for the production of metal-second phase composites may be broadly categorized as powder metallurgical approaches, molten metal techniques and internal oxidation processes. The powder metallurgical-type production of such dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy techniques are then used to form the final composite. Conventionally, however, the ceramic component is large, that is, greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials, because their production is energy intensive, time consuming and costly in capital equipment. Furthermore, production of very small particles inevitably leads to contamination at the particle-to-metal interface which in turn comprises the mechanical properties of a resultant composite. Also, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect ceramic particles from molten metal during molten metal infiltration and to improve bonding between the metal and ceramic. Techniques such as this have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is suitable for large particulate ceramics (for example, greater than 1 micron). The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the interstices. Such a technique is illustrated in U.S. Pat. No. 4,444,603 to Yamatsuta et al. Because this technique necessitates coating the ceramic particles, molten metal handling, and the use of high pressure equipment, molten metal infiltration has not been a practical process for making metal-second phase composites, particularly for making composites of submicron particles where press size and pressure needs would be excessive and unrealistic.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. Although this technique is limited to relatively few systems, because the two metals must have a wide difference in chemical reactivity, it has offered a possible method for dispersion hardening. However, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness and the like.
The presence of oxygen in ball-milled powders used in prior art metallurgy techniques, or in molten metal infiltration, can result in a deleterious layer, coating, or contamination such as oxides at the interface of ceramic and metal. The existence of such layers will inhibit interfacial binding between the ceramic phase and the metal matrix, adversely effecting ductility of the composite. Such weakened interfacial contact may also result in reduced strength, loss of elongation and crack propagation.
Because conventional processes have difficulties, preparation of metal-second phase composites with second-phase dispersoids for commercial uses has not been extremely costly.
In recent years, numerous ceramics have been formed using a process termed "self-propagating high-temperature synthesis" (SHS). It involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. The SHS process involves mixing and compacting powders of the constituent elements and igniting a portion of a green compact with a suitable heat source. The source can be electrical impulse, laser, thermite, spark, etc. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits use of sudden, low powder initiation of high temperatures, rather than bulk heating over long periods at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al, U.S. Pat. Nos. 3,726,643, 4,161,512, and 4,431,448, among others, hereby incorporated by reference.
Similarly, extensive research and development has been conducted in the area of rapid solidification (RS) processing. Rapid solidification processing effects highly desired forms of alloys. Homogeneous material at or above melt temperatures is subjected to a rapid quench or temperature drop to "freeze" the material to desired micro structure. The rate at which the melt is quenched is in the range of approximately 10.sup.4 .degree. C. per second to 10.sup.8 .degree. C. per second. See, for example, U.S. Pat. No. 4,402,745, hereby incorporated by reference.
Current technological interest in materials produced by RS processing, especially when used to produce fine powders followed by consolidation into bulk parts, may be traced, in part, to problems associated with the chemical segregation that occurs in complex, highly alloyed materials during conventional ingot casting and processing. During processing via slower cooling rates used for conventional casting processes, solute partitioning, that is, macro- and micro-segregation of different alloy phases present in these alloys, and the formation of undesirable, massive particle boundary eutectic phases, can occur. Metal powders produced directly from the melt by conventional powder production techniques, that is, drop tower, inert gas or water atomization of the melt, are usually cooled at rates three to four orders of magnitude lower than those that can be obtained by RS processing. The latter removes macro-segregation altogether and significantly reduces spacing over which micro-segregation occurs, if it occurs at all.
Design of alloys made by conventional slow cooling processes is primarily dictated by the corresponding equilibrium phase diagrams. Alloys prepared by such processes are in, or at least near, equilibrium. The advent of rapid quenching from a melt has enabled divergence from equilibrium and has added new alloys with unique structures and properties for commercial use.
Rapid quenching, or rapid solidification, techniques are known for manufacture of metal powder for powder metallurgical (PM) purposes by finely "atomizing" molten metal. Here, RS occurs not by contact but "inflight". This similarly permits little time for particle growth. The small drops produced solidify to form small granules, each one of which essentially constitutes an "ingot" of the molten metal. These small granules can be charged into a container that is evacuated and sealed. Afterwards, the small granules are compacted and concurrently or subsequently heated. This compaction and heating joins together the granules into a solid metal compact of the molten metal composition. This method is valuable for producing homogeneous materials from melt alloys which, if conventionally processed, would result in large-scale heterogeneities and segregation. Additionally, RS can produce materials containing fine metastable dispersoids and second phases.
Prior art techniques for "atomizing" molten metal have included impingement, melt spinning, and nozzle atomization.
In impingement techniques, atomization of molten metal into small drops is usually brought about in inert gas, such as argon or nitrogen. The gas impinges as high speed jets upon a pouring stream of molten metal. Water and steam have also been used. However, water and steam are unsuitable for high speed steel for example, because they cause severe oxidation of granules.
It is also known to atomize a pouring stream by impingement onto a rotating disk to make small drops or "ingots" which then solidify by contact with the surrounding atmosphere, cooling-water or oil bath, or a coolant shower. As mentioned above, in this approach the solidification does not occur by contact with the disk. That contact forms the drops or spears which can have a nearly monosize distribution.
British Patent Specification No. 519,624, hereby incorporated by reference, relates to powdered or granular metallic products constituted of solidified metallic particles derived from molten metal. It also describes a method of producing the product. These solidified metallic particles have spontaneously crystallized from a metastable undercooled state at a predetermined temperature below but close to the freezing point of the metal. The particles have substantially uniform size and composition.
To produce such particles, molten metal is discharged from a suitable receptacle in one or more streams onto a metal surface of such nature that sufficient heat is abstracted from the molten metal to lower its temperature to that of an undercooled state, that is, to a temperature which is slightly below the freezing point of the particular metal but without causing solidification or crystallization. This surface upon which the molten metal impinges can be a belt or a disk rapidly moving either linearly or rotatively, respectively. The molten metal is immediately converted into a stream of film-like proportions on the surface and the extent of the belt or disk surface is such that the molten metal makes contacts therewith for a period just sufficient to undercool it as above defined. Then the molten metal is caused to leave the supporting surface and to continue its travel in the same direction and at substantially the same speed for a sufficient distance to cause solidification. Because the undercooled stream of film-like proportions has little or no integrity, it immediately breaks up into a myriad of fine, small liquid particles which solidify to form a powdered metal.
These operations may be carried out in a vacuum or suitable atmosphere, and the myriad of fine, small liquid particles may pass through a coolant to hasten solidification of the particles or to reduce the distance needed for solidification. During solidification, surface tension causes the particles to assume a substantially spherical shape.
One known rapid solidification technique involving a centrifugal atomizing process is taught in U.S. Pat. Nos. 4,025,249 and 4,343,750, hereby incorporated by reference. It uses forced convective cooling of molten droplets to achieve cooling rates on the order of 10.sup.5 -10.sup.6 .degree. C./sec. This rapid solidification state is designated RSR. Such a RS technique, in conjunction with powder metallurgy techniques for consolidation of the rapidly solidified powders, has produced materials with metastable phases, very fine grain structures, high room-temperature strength and good high temperature properties up to the point of instability of the metastable phases.
An approach to further enhance certain material properties is to blend the RS powder with ceramic powders prior to consolidation. This leads to improvement in some mechanical properties, for example, (SiC/Al), such as commercially available SiC/7090, produced by an RS/PM approach is an example of such a material. The difficulty with this approach is that it suffers from property and processing disadvantages inherent to a PM process. These difficulties include a relatively coarse reinforcement (greater than 1 micron) and/or weak metal/ceramic interfaces due to surface contaminants.
One alternative to conventional RS/PM techniques for developing metal-second phase composites is to form the ceramic phase during RS processing. U.S. Pat. No. 4,540,546, hereby incorporated by reference, describes a "Melt Mix Reaction" (MMR) process involving chemically reacting two starting alloys in a mixing nozzle in which a melt mix reaction takes place between the chemically reactable components of starting alloys to form submicron particles of the resulting compound in the final alloy. The mixing and chemical reaction is performed at a temperature which is at or above the highest liquidus temperature of the starting alloys but which is also substantially below the liquidus temperature of the final alloy, and as close to the solidus temperature of the final alloy as possible. While dispersion-strengthened alloys can be produced by this technique, there appear to be a number of inherent difficulties. First. processing is technically complex. Second, efficient mixing is important if fine dispersions are to be consistently produced. Lastly, very high degrees of superheat will be required to completely dissolve the RS alloying elements in order to produce high loading of dispersoid which necessarily accentuates particle growth, for example, one containing 10-20% dispersoid.
In U.S. Pat. No. 4,240,824, Moskowitz et al, hereby incorporated by reference, describe a process for producing a boron-containing nickel or cobalt spray-and-fuse self-fluxing alloy powder containing an internally precipitated chromium boride or nickel boride. In this patent, starting materials are alloys containing precursors of the hard precipitate, and the melt is precooled to a temperature about 50.degree. F. higher than the viscous temperature prior to atomization. The particles are formed in the secondary atomization step, and are preferably larger than 10-15 microns in average particle size. No teaching is found for precipitating the particulate material prior to the atomization steps, or of precipitate having an average size less than 1 micron.
Narasimhan, in U.S. Pat. No. 4,268,564, hereby incorporated by reference, teaches the preparation of sheets or strips of amorphous metal containing embedded particulate matter, of 1 to 100 micron particle size, by forcing a glass-forming alloy containing particulate matter, formed in-situ, onto a rapidly moving chill surface. This technique was surprising because it had previously been believed that incorporation of particulate matter, especially of wettable particulate matter, into a molten glass-forming alloy would preclude quenching into an amorphous body due to nucleation of crystallization. Further, inclusion of particulate material in the metal melt in a melt spin process has led to rapid plugging of the orifice. The reference does not teach preparation of a rapidly solidified powder having an evenly dispersed particulate material therein. In fact, the reference specifically teaches that the particulate material is concentrated at the surface of the strip material produced.
These prior art techniques produce conventional powdered metal products.
The present invention overcomes the disadvantages of the prior art noted above including current rapid solidification technology. More particularly, the present invention permits simplification of procedures and equipment compared to the prior art. For example, the present process obviates the need for multiple furnaces and mixing and control equipment because ceramic is previously precipitated in-situ. The present invention also overcomes the need for forming multiple melts of components at very high melting temperatures. Further, high loading composites can be prepared without the necessity of achieving high levels of superheat in RS holding furnaces. One advantageous embodiment of the present invention is that two or more dispersoids may be introduced into the metal powders, e.g., one previously preformed by the process of the present invention to enhance modulus and a second resulting from conventional RS processing to increase strength. Thus low temperature strength can be enhanced by the latter and high temperature strength enhanced by the former. The present invention includes a process for producing a metal matrix material suitable for subsequent atomization, whereas conventional metal matrix composites are not believed suitable for RS atomization. Applicants' invention also provides for a cleaner particle/metal interface compared with conventional metal ceramic composites made by PM techniques using RS powders because the reinforcing particles are formed in-situ. This leads to a superior product.
This invention may also result in improvement from incorporation of a stable dispersoid into the composite which extends the high temperature working range of the composite relative to conventional RS composites that typically contain metastable phases. Moreover, incorporation of dispersoid prior to RS may provide surfaces for precipitation, consequently, a more efficient precipitation of metastable rapid solidification phases. In some cases, for example, titanium-based alloys, the addition of rare earth elements, like cerium or erbium, to the dispersoid-containin melt may result in improved scavenging of interstitials such as oxygen, leading to the formation of an additional oxide dispersoid and effective deoxidation of the matrix alloy. With these facts in mind, a detailed description of the invention follows which achieves advantages over known RS processes.